Brake pressure apply

ABSTRACT

A method for brake pressure apply in a hydraulic brake system includes commanding a cage clearance reduction phase and commanding a wheel control phase subsequent to the cage clearance reduction phase. Accordingly, the cage clearance is reduced prior to entering the wheel control phase. A method for cage clearance reduction in a hydraulic brake system for roll stability control is also provided.

TECHNICAL FIELD

The present invention is related to a method for brake pressure applyand more particularly directed to brake pressure pulsing to achieveerror reduction in a brake pressure command.

BACKGROUND

Brake controllers typically use pressure control as part of antilock,traction and/or stability control systems. The accuracy of the pressurecontrol is in part affected by the compliance within the brake controlsystem, such as the compliance of each brake connected to the hydraulicsystem as controlled by the electronic controller. The compliance isalso influenced by pressure versus volume relationship as manifestedwithin the brake hydraulic system when actuating the mechanicalconnection by the hydraulic. Resultantly, variation in the pressureversus volume relationship, including other compliance effects, mayexhibit minor changes such as in a pressure control estimation employedwhen using pressure control. Accordingly, it is desirable to improve thepressure control estimation by refining control or responsiveness.

Representatively, FIGS. 1 and 2 show a graph of pressure control for arear left brake having a 0.3 mm cage clearance and a 0.7 mm cageclearance, respectively. Cage clearance is generally a measure of thelinear difference between the outer diameter of the shoes and the innerdiameter of the drum when there is no pressure control acting on thebrakes. A pressure control estimate 11 is determined for a particularvehicle by modeling or by empirical testing and determination, wherebypressure control may be achieved by implementing a pressure controlrequest 12 in a brake controller in order to achieve the desiredpressure control estimate. When the brake controller implements thepressure control request 12, an actual pressure control of the brake isachieved as manifested by a pressure control measured response 13. It isrecognized that the actual response 13 is determined by utilizing apressure sensor in a test vehicle and may not be observable in a vehicleutilizing the invention to advantage because the invention necessarilyeliminates the need for pressure transducers otherwise required forpressure control. The pressure control measured response 13, whencompared to the desirable pressure control estimate 11 as shown in FIG.1, results in a 26.5% mean pressure estimate error 14. The mean pressureestimate error is the average of the error between the actual pressureand the estimate pressure. The pressure control measured response 13,when compared to the desirable pressure control estimate 11 as shown inFIG. 2, results in a 267% mean pressure estimate error 15.Illustratively, only a 0.4 mm change in cage clearance causes anadditional 241% error in the pressure control. Therefore, changes incage clearance can affect braking performance and pressure control.

Therefore, there is a desire to provide refined pressure control withmore accurate pressure estimation.

SUMMARY

Accordingly, a method for brake pressure apply in a hydraulic brakesystem is provided. The method advantageously reduces the effects ofcage clearance, including the associated cage clearance dynamics.

A method for brake pressure apply in a hydraulic brake system includescommanding a cage clearance reduction phase and commanding a wheelcontrol phase subsequent to the cage clearance reduction phase.Accordingly, the error caused by cage clearance is reduced in the wheelcontrol phase.

A method for cage clearance reduction in a hydraulic brake system forroll stability control is also provided.

Other advantages and features of the present invention will becomeapparent when viewed in light of the detailed description of theembodiments when taken in conjunction with the attached drawings andappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of pressure control for a rear left brake having a0.3 mm cage clearance.

FIG. 2 shows a graph of pressure control for a rear left brake having a0.7 mm cage clearance.

FIG. 3 shows a graph of pressure control for a rear left brake, having a0.7 mm cage clearance, utilizing a cage clearance reduction phase forenhancing roll stability control.

FIG. 4 shows a graph of pressure control for a rear left brake, having a0.3 mm cage clearance, utilizing a cage clearance reduction phase forenhancing roll stability control.

FIG. 5 is a block diagram showing a method for RSC in a brake controlsystem according to a first embodiment of the invention.

FIG. 6 is a block diagram showing a second embodiment of a method forcage clearance reduction phase according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention provides a method for refined pressure control. Inparticular, the invention provides a refinement to pressure control whenaffected by differing cage clearance or different pressure versus volumerelationships required to bring a shoe into braking contact with a drumin order to achieve a desired pressure control. While pressuretransducers may be utilized for pressure control, it is desirable forthe above-mentioned reasons to eliminate or not require the pressuretransducers. Therefore, in order to have pressure control, a pressureestimate is implemented by the controller, such as the pressure estimateutilized by a roll stability control (“RSC”) system for each wheel ofinterest.

Cage clearance is generally defined as a measure of the lineardifference between the outer diameter of the shoes and the innerdiameter of the drum when there is no pressure control acting on thebrakes. Optionally, cage clearance may be defined be as the differencebetween a brake drum and a brake shoe. Furthermore, cage clearance maybe defined as the mechanical force required to bring a brake drum intocontact with a brake shoe.

The pressure estimate is a function of a fluid flow estimate. The fluidflow estimate is a function of the pressure estimate. Because the fluidflow estimate is explicably tied to the pressure estimate, carefulexecution of the pressure estimate by the controller is required ifcontrol is to be appropriately achieved. Adding to this complexity, drumbrakes are less reactive when controlled by a controller using thepressure estimate when compared to a system using feedback pressurecontrol. Drum brakes are sensitive because of variation in the cageclearance and the associated cage-drum dynamics. The cage clearancevariation is manifested primarily by changes in the volume of fluidrequired or consumed when moving the shoe against the drum. The dynamicsvariation is manifested in the operating region where the frictionsurfaces of the drum and shoe are not in contact with each other.

Additionally, the pressure estimate is affected by the brake'svolumetric stiffness. For example, the volumetric stiffness of the shoewhen it is not in contact with the drum may be on the order of 4-8bar/cm3, while volumetric stiffness after the shoe contacts the drum maybe on the order of 400-800 bar/cm3 (these degrees of magnitude will varydepending upon the application). As a result, when fluid is pumped intothe brakes, the pressure rise is greater per unit volume after the shoemakes contact with the drum. Conversely, when a release valve holdingthe pressure in a given wheel is opened, the fluid release is at agreater rate when the drum and shoe are still pressed against eachother.

Furthermore, the pressure estimate is affected by the cage-drumdynamics. The drum brakes are affected by a self-energizing actioncaused by the rotation of the drum relative to the shoe. Thisself-energizing action introduces a force component that acts to helppress or hold the shoe against the drum. This self-energizing action isovercome as the inertial effects decrease as the shoe returns to itsnon-contact or resting clearance position. The inertial effect isinfluenced by the pressure release, i.e., the compliance effect ofpressure versus volume relationship in the pressure release valve asdescribed in U.S. patent application Ser. No. 11/381,166 filed May 2,2006, by the same assignee, entitled “METHOD TO ACHIEVE PRESSURE CONTROLROBUSTNESS IN BRAKE CONTROL SYSTEMS,” incorporated herein by reference.

Taking advantage of the observed behavior allows for control of thepressure command to reduce the effective clearance between the shoe andthe drum of a brake prior to exerting control over the wheel ofinterest. As a result, the variation in the pressure model due to thedynamics of the drum brake system in the region of operation where theshoe is not in contact with the drum may be significantly reduced.Accordingly, a pressure profile may be designed by taking advantage ofthe dynamics mentioned above, thereby allowing for pressure control of ahydraulic brake system, such as an RSC, without the added cost andcomplexity explicably required when using pressure transducers in orderto reduce error.

FIG. 3 shows a graph of pressure control for a rear left brake, having a0.7 mm cage clearance, utilizing a cage clearance reduction phase 21 forenhancing roll stability control. The brake controller enters the cageclearance reduction phase 21 prior to entering a wheel control phase 22by supplying a pressure control request 24, which would substantiallyresult in a pressure control estimate 25 if the compliance or cageclearance dynamics 20 were overcome. A measure of the actual pressurecontrol expected is shown by a representative pressure control measuredresponse 26. The brake controller may then enter the wheel control phase22, wherein the commanded pressure control request 24 results in thedesired or measured response 26 approximating the control estimate 25because the cage clearance dynamics 20 have been minimized by the cageclearance reduction phase. The wheel control phase 22 is followed by aninertial phase 23 in which the cage clearance dynamics 20 are overcomeuntil the inertia of the system dissipates. It is also recognized, thatthe cage clearance reduction phase 21 inherently includes inertialphases depending upon the type, size and duration of pressure builds andreleases while achieving cage clearance reduction error. Accordingly,the wheel control phase 22 may follow the cage clearance reduction phasewithin a given time lag to achieve the same effectiveness.

Like FIG. 3, FIG. 4 shows a graph of pressure control for a rear leftbrake, but having a 0.3 mm cage clearance, utilizing a cage clearancereduction phase 21 for enhancing roll stability control. A pressurecontrol response 29 is achieved by implementing a pressure controlrequest 27 to obtain a pressure control estimate 28 in the brake controlsystem by first entering the cage clearance reduction phase 21 beforeentering the wheel control phase 22. While the cage clearance differs by0.4 mm in FIGS. 3 and 4, the mean pressure estimate error issubstantially reduced, thereby minimizing the error effect in open looppressure control caused by differing cage clearances.

Accordingly, the effective drum brake cage clearance reduction phasepromotes closer agreement between the pressure estimate and the actualpressure. It is expected that the cage clearance reduction phasepressure profile may be used throughout the operating range of the cageclearance. When the results are compared for the 0.7 mm cage clearanceto the 0.3 mm cage clearance, the difference in the pressure estimateerror is approximately 3.7 percent during the wheel control phase. Thisillustrates that the brake hydraulics, for this wheel, experiences apronounced increase in volumetric stiffness earlier in the effectivecage clearance reduction phase when there is less clearance between itand the drum. Consequently, the drum brake experiences significantlyhigher pressure during this phase for a similar amount of fluid flow.This may be managed in part by releasing pressure in the brake for astrategic amount of time prior to entering the wheel control phase. Whenthe valve is opened to release pressure in the brake, the pressure dropssubstantially quicker than when the same action was commanded with theshoe at a higher cage clearance because there is more pressure in thebrake acting against a relatively stiff influence. As a result, thepressure in the brake and the clearance between the shoes and the drumare closer over the range of initial cage clearance when a pressureincrease is subsequently commanded during the wheel control phase.Advantageously, there may be additional time lag before the shoes returnto their initial cage clearance due to the self-energizing effectbetween the friction surfaces when there is relative motion between theshoes and the drum.

This illustrates that a series of pressure commands, i.e., pressurebuilds or releases, may be fashioned to reduce the effective cageclearance and therefore reduce the error of the pressure estimate. Thedesign of a pressure command in the effective cage clearance reductionphase coordinated with another pressure command in wheel control phaserequired for RSC allows a usable pressure estimate to be advantageouslyutilized during the wheel control phase even when there is variabilityin the cage clearance.

FIG. 5 is a block diagram showing a method for RSC in a brake controlsystem 10 according to a first embodiment of the invention. Theinvention is presented for convenience for use with RSC but may be usedto advantage in any other hydraulic brake system. Initially, after theRSC is initiated, a cage clearance reduction phase 30 is commanded. Thecage clearance reduction phase 30 may include a pressure build orincrease followed by a pressure dump or decrease, or series of pressurebuilds and or dumps, wherein the cage clearance is reduced or minimizedprior to receiving a pressure command in the wheel control phase 31. Thecage clearance reduction phase 30 takes advantage of the compliance orcage clearance dynamics mentioned above to neutralize or diminish theeffect of changing cage clearances, thereby achieving improved pressurecontrol response in response to the pressure control request premisedupon the pressure control estimate when executing the wheel controlphase 31. After the wheel control phase 31 is commanded, a decisionblock 32 is entered to determine whether the RSC is complete and if so,the RSC is completed for the wheel of interest. If the decision block 32is not completed, then decision block 33 is executed, wherein adetermination of whether the inertial phase is exceeded. If the inertialphase is exceeded the method is returned to the cage clearance reductionphase 30. If the inertial phase is not exceeded, i.e., the clearance isstill substantially reduced due to inertial effects, the method isreturned to the wheel control phase 31, wherein RSC may again beapplied.

It is recognized that duration and magnitude for each pressure build isdetermined by testing for a particular vehicle and implementable in thebrake control system according to the invention. Also, the inertialphase duration may be empirically determined for a particular vehicle.

Optionally, decision block 33 may be implemented prior to entering thewheel control phase 31.

It is also recognized that the brake control system 10 may beimplemented at different times and in different ways for each wheel,independently.

FIG. 6 is a block diagram showing a second embodiment of a method forcage clearance reduction phase 40 according to the invention. Afterentering the cage clearance reduction phase 40, a pressure build 41 iscommanded. The pressure build 41 is a pressure step having magnitude andduration, but it is recognized that the pressure build may have anyother form such as a ramp function, without limitation, that would berecognized by a person of skill. After the pressure build 41 iscommanded, decision block 42 may be entered to determine whether a wheelcontrol phase is required, and if so, the cage clearance reduction phaseis completed and the brake control system continues. If the wheelcontrol phase is not required, then decision block 43 may be entered todetermine whether the cage clearance is minimized. The cage clearancereduction phase is returned to the command pressure build 41 if the cageclearance is not minimized. If the cage clearance is minimized then thecage clearance reduction phase continues to a command pressure release44.

The pressure release 44 is commanded to provide pressure reduction inthe brake from the command pressure build thereby minimizing brakingforce affect before it is required by the RSC, while taking advantage ofthe inertial and compliance effects within the brake to decrease cageclearance. The pressure release 44 is a pressure step having magnitudeand duration, but it is recognized that the pressure release may haveany other form such as a ramp function, without limitation, that wouldbe recognized by a person of skill. After the pressure release 44 iscommanded, decision block 45 may be entered to determine whether a wheelcontrol phase is required, and if so, the cage clearance reduction phaseis completed and the brake control system continues. If the wheelcontrol phase is not required, then decision block 46 may be entered todetermine whether the cage clearance is minimized. The cage clearancereduction phase is returned to the command pressure build 41 if the cageclearance is not minimized. If the cage clearance is minimized then thecage clearance reduction phase is completed and the brake control systemcontinues.

Optionally, decision blocks 43 and 46 may be determined by testing toobtain the criterion for a particular vehicle that is then implementedin the RSC to determine whether the criterion is met. The criterion may,for example, be determined by satisfying particular thresholds such ascommanded pressure build, pressure release, pressure duration orinertial lag, without limitation. Also, decision blocks 43 and 46 may bedetermined by utilizing a sensor to determine cage clearance, bymeasuring fluid displacement rate in the brake system, or by monitoringthe change in volumetric stiffness in the brake system.

It is to be understood that the configuration for pressure build andrelease will depend upon the particular system to which it isimplemented. In this regard, generally, the invention may be tailored tovarious drum brake designs and implemented for mass production bydesigning and/or calibrating the pressure request to the drum brakesystem of interest. Calibration may be accomplished by measuring theactual pressure in the drum brake assembly or by measuring the cageclearance during test pressure commands. The pressure commands may be afunction of time and/or the pressure estimate.

The calibration to determine the builds or pulse train required forimplementation may be designed to determine as follows: 1. Pressureincrease command designed such that the shoe comes into contact with thedrum at the maximum expected cage clearance. At the same time, thispressure command should not be intrusive to the driver when the drumbrake system is at its minimum expected cage clearance. 2. Pressuredecrease command such that the inertia of the shoe assembly would notallow it to reach the maximum expected cage clearance during the releasebefore more brake fluid is pumped for the subsequent build phase,resulting in a reduced effective cage clearance.

Alternatively, the method above may include measuring the cage clearanceand using this signal to calculate the fluid flow needed to bring theshoe in contact with the drum or until there is a significant change inthe volumetric stiffness. Also, the shape of the pump current may beused to estimate when the shoe has made contact with the drum.

While particular embodiments of the invention have been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

What is claimed is:
 1. A method for brake pressure apply in a hydraulicbrake system comprising: commanding a cage clearance reduction phase asa series of pressure increases and decreases while a measured pressureis greater than zero and until the measured pressure matches a pressurecontrol estimate during the cage clearance reduction phase; andcommanding a wheel control phase subsequent to the cage clearancereduction phase, wherein a cage clearance is reduced prior to saidcommanding of the wheel control phase.
 2. The method of claim 1 whereinthe cage clearance reduction phase and the wheel control phase areinitiated during a roll stability control (“RSC”).
 3. The method ofclaim 2 further comprising determining whether the RSC is complete. 4.The method of claim 3 further comprising determining whether an inertialphase is exceeded when the RSC is not complete, the inertial phase beinga phase during the cage clearance reduction phase during which time abrake system inertia is dissipated.
 5. The method of claim 4 whereinsaid commanding the cage clearance reduction phase is in response tosaid determining whether the inertial phase is exceeded.
 6. The methodof claim 1 further comprising determining whether an inertial phase isexceeded, the inertial phase being a phase during the cage clearancereduction phase during which time a brake system inertia is dissipated.7. The method of claim 6 wherein said commanding the cage clearancereduction phase is in response to said determining whether an inertialphase is exceeded.
 8. The method of claim 6 wherein said commanding thewheel control phase s in response to said determining that the inertialphase is not exceeded.